[hotfix] Fix a bug which throw exception when no rack was configured (#986)

Author: zcoo

fluss-client/src/main/java/com/alibaba/fluss/client/utils/MetadataUtils.java

@@ -298,7 +298,7 @@ private static Map<Integer, ServerNode> getAliveTabletServers(MetadataResponse r
                                             serverNode.getHost(),
                                             serverNode.getPort(),
                                             ServerType.TABLET_SERVER,
-                                            serverNode.getRack()));
+                                            serverNode.hasRack() ? serverNode.getRack() : null));
                         });
         return aliveTabletServers;
     }